Grjótagjá is a small lava cave near Lake Mývatn in Iceland with a thermal spring inside. In the early 18th century, an outlaw named Jón Markússon used the cave for bathing. It was a popular bathing site until the 1970s when the water temperature rose above 50°C due to volcanic eruptions. The nearby Stóragjá cave is now used as an alternative bathing spot. Grjótagjá was even featured in an episode of Game of Thrones.